World's Most Powerful NERF Blaster vs Watermelon

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[hammerman]

Is the foam ball closed cell foam? Do you think the foam being inside the vacuum chamber could be expanding the foam and causing increased drag on the barrel? I would love to see the same tests with something incompressible (or even a 3d printed slug)

[PhilosopherRex]

Not sure about the vacuum, but it seems like the too high air pressure was causing the ball to deform in the barrel creating extra friction between the ball and barrel, so reducing velocity.

[CraftComputing]

My recommendation is to actually shorten the barrel. I've seen a number of vacuum cannons like this, and there's a very weird relationship between the length of the barrel and how quickly the air will get by your projectile. Since you're shooting foam balls, I'm guessing they're deforming in the barrel enough to let air escape around them, decelerating the balls quite a bit before exiting.

A shorter barrel may make it so the air never makes it in front of the ball, leading to a faster exit velocity with a vacuum.

Good luck man!

[backtosquare]

Here my two cents:
1) Reduce barrel diameter to gain a better seal between the ball and the barrel even under high pressure
2) add lubricant to the barrel to reduce surface friction
3) reduce length of barrel to reduce friction
4) substitute the burst flap with a rubber flap, an hinge and a spring that keeps the flap in a unstable balance condition to allow the flap to slam open as soon as the little bubble of air in front of the ball pushes it.

[stephenwhite]

One idea I would have is to replace the ball valve with a butterfly valve. This would increase the speed at which the air pressure is released, giving you greater force and therefore greater starting velocity. (Thanks to Punkin' Chunkin' for this one!)

[namewarvergeben]

At 15:25, the bulge in the tape looks yellow and bounces a little. It could be the ball itself stopping dead on the tape before enough air-pressure builds up behind it to launch it again from there?

And at the time when the ball impacts the tape it has already cleared the vacuum-trap. So the air-pressure behind it might actually be decreasing for a fraction of a second while the air takes a detour into said vacuum-trap

[TheHookUp]

Maybe the higher air pressures are able to get around the ball since it isn't rigid? Pulling a vacuum is useful, but maybe you need to be able to close off that chamber after the vacuum is pulled so it doesn't get re-pressurized by the air that is supposed to be accelerating the ball?

[crashfocusfilms]

Put one of the balls in a pressure pot so you can see how much it deforms under a vacuum. my guess is what everybody else is already saying the ball isn't rigid enough too not deflect under the pressure(s).

[carlfeynman]

No matter how much pressure you apply, the air will never go faster than Mach one at the narrowest point. You can turn pressure into velocity with a converging-diverging nozzle, also called a deLaval nozzle. This will still have Mach one at the throat, but can have higher velocities at the barrel. This is the only way you will be able to get your projectile faster than the speed of sound.

[kleintra]

My first thought was a sabot to really seal off the air from getting around the ball. Sabots used for black powder rifles expand with back pressure so that the higher the pressure the tighter the seal against the barrel. I'm not sure of the material they are made from but something similar to delrin comes to mind. There would be the downfall of a sabot leaving the barrel at 1000+kmh.

[ruudhooff]

High pressure causing a deformed ball that slows itself down due to friction against the wall seems most logical to me. Could be applicable at both scenarios (too high pressure and the vacuum tape break). Guess it needs to be tested with a solid ball 🤓.

[LarsKristian]

Nice project.
One theory I have not seen many comments on, is that the ball will change location inside the barrel after pulling a vacuum.
I assume the ball is a tight fit, so when loading the ball some air will get trapped between the ball and the valve.
When pulling a vacuum the ball expands and it gets pushed towards the tip of the barrel. Making the vacuum ineffective and the barrel only 4-5cm long.

You can easily check this by using a mirror to look inside the barrel. Never place your head in front of this thing, it is a disaster waiting to happen.
If this is the case you should get the same performance as only using air pressure and a 4-5cm long barrel. Could be a nice visual demonstration for a video.

Also consider changing the color of the barrel tip. The color orange usually signals this is somewhat not dangerous. ;)
Take a look at "piston vacuum cannon" super simple design and awesome performance.

[michaelkean]

There are a few things that could be causing the measured speed drop as you increase the pressure; some are going to be more complicated to test for than others.
Valve friction: As you increase the pressure you'll deform the valve seals increasing the resistance to opening, slowing the valve down. Point the high speed camera at the valve arm and fire a couple of shots at different pressures. A slow opening valve is more than capable of off-setting the gains from the increased pressure. You could use a stronger spring or apply more leverage to overcome the increased friction but it maybe easier to go for a fast actuating valve (a lot of compressed air potato guns used to use air operated sprinkler valves)
It is also probably worth trying to get the muzzle in shot so you can see if the valve reaches full open before the nerf ball clears the end of the barrel. If the valve isn't open fully well before the ball leaves the barrel then you could always try making the barrel longer, that said the barrel already looks quite long for the diameter of the ball.
Barrel friction: The increased pressure could be deforming the ball a 2.3cm ID tube has a cross sectional area of about 2/3rds of a square inch so you'll have about 10lbs of push on that ball for every 1bar of pressure so at 60bar you are well over 250kg pressing on a tiny foam ball, so I assume it'll deform and grip the barrel tighter. That said the additional barrel friction is probably fairly minor given the quantity of pressure behind it. Repeating the pressure tests with a more rigid projectile should confirm this but if you get a solid projectile jammed in the barrel things could get quite unpleasant. Just lubricating the ball may help although don't use anything remotely flammable as you could get some unexpected extra energy if it diesels in the barrel.
The vacuum issues could be pulling the ball up the barrel if it isn't seated hard against the valve, giving it less time to accelerate and creating a vacuum behind the ball that cushions the 'shock' of the valve opening, or the tape barrel cap you used being a bit too strong. Most of the vacuum assisted launchers I've seen seemed to have a bit of a looser fitting projectile rather than being a tight fit like a traditional bullet, that could have been an accident rather than design though.

[chod]

Alexandre Chappel: "I Built The Worlds Most Powerful Nerf Blaster"
Smarter Every Day: "Are you sure about that?"

[SpencerPaire]

The vacuum may be slowing the shot because that closed-cell foam used in Rival balls will expand in vacuum, creating too much friction on the barrel. I'd test this by using an inelastic tape (like masking tape, if it's strong enough) to bind the ball down to a lesser diameter, or just by experimenting with a rigid projectile.

The increased pressure hurting performance may be due to wacky aerodynamics causing choke as the air tries to pass through the valve. Strange things can happen at very high pressures. Or it could be hitting the ball so hard that it tries to mushroom up, again pressing against the barrel.

Another commenter mentioned that air trapped behind the ball may expand when the vacuum is drawn, pushing the ball down the barrel after its sealed up, so adding a tiny little bypass around the ball should keep this from happening. Just make sure it's a pinhole, or use some metal foam so that when the high pressure is added, it doesn't bypass the ball.

Finally, one more idea. Those ballistic chronographs should be extremely accurate, even up to supersonic speeds, but they are sensitive to environmental lighting; the 50-60hz of normal lights (especially LED) can interfere with the light gates they use to measure speed. Try shrouded it or using it outside or using the attached lights to see if you can get more consistent data (just note that rival can vary by a couple m/s from shot to shot.)

Very cool build! So glad we got to see the construction and the issues.

[kkuhn]

Biggest area of improvement is to remove the "vacuum reservoir". I suspect that this is actually diverting the high pressure just as the ball passes it and then the ball is slowed rapidly (due to its low inertia) through friction with the barrel and bursting the membrane. Since it is so close to the end of the barrel, the ball doesn't accelerate again after slowing. Additionally, the valve is probably too slow. You should look at how actual airgun valves are designed and use that style of valve.

[BananaDude]

without watching, i have some notes *THIS IS BEFORE I WATCHED IT DONT GET MAD*
1. Dont use a vacuum barrel, it will deform the projectile since nerf/toy blaster projectiles are soft
2. Please use a De Laval nozzle inside after a compressed air-tank (not the end of the barrel), it lets a nozzle compress and go fast, but limits at the speed of sound, using the De Laval part, like an anti-nozzle almost, it will let is go faster.

edit 1: well there goes note 1, a vacuum barrel it is...
edit 2: well, forget the De Laval nozzle aswell
edit 3: so the vacuum barrel didnt work, i was right!

edit 4: so this is a separate idea, possibly replace the ball valve with a check valve going with the pressure, it means you can drop the spring and its MUCH faster to open, the air pressure opens it.
also drop the tape and vacuum barrel

[samuelgunnarsson]

1: Youll probably need a solenoid valve, it dumps the energy WAY faster
2: The ball is probably to soft, and hence the fit too loose, and therefore youll need a padding around/behind the projectile to seal up the fit properly

[MakenModify]

I would bet that the squishy projectile is the problem. 2 possibilities: the pressure from the back is so high, that it swishes the ball so hard that friction to the tube becomes your main loss. Or the air squeezes around the ball and the ball looses speed. This second theory would fit the observation with the vacuum. Now even more air squeezes around the ball, fills the vacuum tank and decelerates the ball due to touch pressure from the front. Anyways, awesome video, looking forward to an update ;)

[BirroTheBear]

Have you considered what happens to the foam ball when in a vacuum? My guess is that the reason for greater speed whit out the vacuum is probably because the foam ball expands in a vacuum and is breaking its speed due to friction in the pipe on the way out.